Metaschema processing index

[ahwagner]

Metaschema processor was built on top of a self-described import model, where files point to other files in the repository space.

With our recent build restructure, this has proven to be burdensome, requiring brittle, repo-specific path management.

It would be useful to have each repository create a simple index of import source-file names -> filepaths relative to the index, that GMP could load. Imports and $refCuries alike would be configurable as linked entries.

It would also be cool to use the python-jsonschema/referencing library to support online ref-retrieval, once the GKS specs are a little more stable and we have ref resolution set up through w3id.